This year's Google I/O is still fresh in everyone's memory, skydiving hijinks helped with that, but Google is already thinking about next year's edition. In fact, it already announced the date for Google I/O 2013, it's going to be earlier this year, in May.
The 2012 edition was supposed to be held in April, but was pushed to June to enable it to last three days. This year, Google announced from the get-go that the developer conference will last three days.
Registration for the conference will open in February most likely, Google will provide more details at that time. Tickets sold out in 20 minutes this year so be sure to keep an eye out if you want to attend.
Each year Google I/O gets bigger and bigger, so it's safe to assume that interest is higher than ever. As for what's in store for this year's edition, it's pure speculation at this point, but some things are pretty certain.
You can expect the next major Android update, Android 5.0 codenamed Key Lime Pie for now to be introduced at Google I/O. Android 4.1 Jelly Bean, the last named version, landed at this year's I/O.
While you can't exclude another update, something like Android 4.3, by then, it's very likely that the major update will be held off until I/O 2013.
Other things you can expect include some updates to the Nexus line, perhaps a new Nexus 7 since the tablet will be close to one year old by then.
Google showed off the Q, which it later pulled, and shared some updates on Project Glass this year. If things go well, you can expect a much bigger focus on Glass, maybe even some prototype devices for developers, as was promised this year. Also a possibility, another skydive or something even bigger if Google doesn't want to repeat itself.